Direct sequence code division multiple access (DS-CDMA) communication systems have been proposed for use in cellular telephone systems with traffic channels located at 800 MHz and in the personal communication system (PCS) frequency band at 1800 MHz. In a DS-CDMA system, all base stations in all cells may use the same radio frequency for communication. One known DS-CDMA system is defined in Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) Interim Standard IS-95, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" (IS-95).
In addition to traffic channels, each base station broadcasts a pilot channel, a synchronization channel, and a paging channel. The pilot channel or pilot signal is a pseudorandom noise or PN code. The pilot channel is commonly received by all mobile stations within range and is used by the mobile station for identifying the presence of a CDMA system, initial system acquisition, idle mode hand-off, identification of initial and delayed rays of communicating and interfering base stations, and for coherent demodulation of the synchronization, paging, and traffic channels.
The pilot signal transmitted by each base station in the system uses the same PN code but with a different phase offset. The base stations are uniquely identified by using a unique starting phase or starting time for the PN sequences. For example, in IS-95, the sequences are of length 2.sup.15 chips and are produced at a chip rate of 1.2288 Mega-chips per second and thus repeat every 262/3 milliseconds. The minimum time separations are 64 chips in length allowing a total of 512 different PN code phase assignments for the base stations.
At the mobile station, the received RF signals include pilot, synchronization, paging, and traffic channels from all nearby base stations. The mobile station must identify all the pilot signals that are receivable including the pilot signal from the base station with the strongest pilot channel. In prior art mobile stations, a correlator has been used as a receiver pilot searching element to serially search for the PN phases of the receivable pilots. The received PN phase is correlated with system PN codes generated in the mobile station. Knowledge of the correct PN phases of the base site(s) with which the mobile station communicates allows the coherent detection of all the other channels transmitted by the base station. Incorrect PN phases will produce a minimal output from the correlator.
Because the PN sequence phase space is large, the prior art serial, real time, correlation technique has taken a prohibitively long time to correctly locate pilot signal energy. At a minimum, with strong signals, system acquisition upon powering up the mobile station can take up to 2.5 seconds or more. With no receivable pilots present, the mobile station will continue to search the entire phase space of the PN sequences until a system time out occurs, which may be 15 seconds. Then the mobile station moves to another RF frequency and again attempts to acquire the CDMA system. The searching process is repeated on subsequent frequencies until a pilot signal is found.
The long time delay in system acquisition is inconvenient and undesirable for most users. A user turning on a radiotelephone expects to be able to use the radiotelephone immediately, with minimal delay. A delay of even 2.5 seconds is too long for many users and longer delays could have serious consequences, for example, for emergency "911" calls.
The prior art pilot channel searching method creates further limitations for all of the other uses of the pilot channel after initial system acquisition. Typical DS-CDMA mobile station receivers utilize a rake receiver having three or more independently controlled fingers which are time aligned to the correct PN sequence phases as determined by the receiver pilot phase searching element. The rake fingers are normally assigned to the strongest rays received from all communicating base stations as determined by the receiver pilot phase searching element. Ray assignments are updated in a maintenance process using the pilot phase searching element information.
If the pilot phase searching element is slow, resulting in slow maintenance of the assignment of the strongest rays to the rake fingers, the receiving performance of the mobile station is degraded under fading conditions. Under certain conditions called "rapid PN," there is a high percentage of dropped calls. The rapid PN problem occurs because the available PN pilot signals are changing so fast that prior art searching elements cannot keep up.
Idle hand-off is the process of attaching to and listening to the paging channel of the base station with the strongest pilot as identified by the pilot searching element. When the mobile station receives a page or accesses the system to place a call, it is important that the mobile station is listening to the page from or tries to access the base station associated with the strongest received pilot. This requires a fast pilot phase searching element, particularly when the mobile station is in motion.
The poor performance of the prior art searching mechanism also affects the soft handoff performance of the mobile station. When in a call on a traffic channel, the pilot searching element is used to maintain the proper rake finger assignments for optimum demodulation of the traffic channel and to identify interfering base sites. If an interfering base site is found, it is reported by the mobile station to the base site as a candidate for soft handoff. Soft hand-off is a DS-CDMA system condition where a mobile station is communicating with more than one base site simultaneously. Pilot signals from adjacent base stations need not be closely located in the pilot phase space. Thus, in addition to speed, the searching element needs to be nimble, that is, able to look across the entire phase space as well as looking only at specific PN offsets.
New requirements for mobile stations will require Mobile Assisted Hard Handoff, or MAHHO, capabilities. In MAHHO, the mobile station changes the frequency of the radio link as it is handed off from one base station to another. Due to the full duplex nature of the CDMA air interface, this requires breaking the radio link, going to another frequency, looking for pilot signals, returning to the original frequency and reacquiring the pilot to reestablish the link. The prior art searching element which requires 2.5 seconds to acquire a pilot is unsuitable for MAHHO purposes.
Another limitation of the prior art involves slotted mode operation. For battery powered portable mobile stations it is also very important to conserve battery charge when waiting for pages. IS-95 provides a slotted mode that allows portable stations to power down except for the periods when their assigned paging slot information is transmitted by the base stations. The paging slot interval can be as short as 1.28 seconds, and periods of 1.28 seconds multiplied by powers of two can be used for more battery savings. During these intervals, the mobile station only needs to monitor the paging channel for up to 160 ms and "sleeps" in a low power mode the remainder of the time.
When operating in slotted mode, a portable station may have to search the phase space of as many as twenty base stations every time it wakes up. To reliably receive the paging slot after waking up, the portable station must be listening to the base station which is providing adequate signal strength. When the mobile station is in motion, the correct base station to decode can easily change from one paging interval to the next paging interval. Therefore it is very important to have a fast pilot searching mechanism to identify the correct base station pilot before the start of the assigned paging slot. Using the prior art pilot searching mechanism requires the portable station to wake up well before the paging slot to allow sufficient time to sequentially search the PN sequence phase space. This negates a substantial part of the potential battery savings afforded by slotted mode.
Accordingly there is a need for a fast and accurate pilot searching mechanism that will improve mobile station performance in the areas of DS-CDMA system identification (service detection), initial system acquisition, idle mode hand-off, soft hand-off, slotted mode operation, and identification of initial and delayed rays of communicating and interfering base stations for the purposes of coherent demodulation of the synchronization, paging, and traffic channels.